The present invention relates to a color image forming apparatus by which a color image is formed in the following manner: when an image forming body is rotated a plurality of times, monocolor images are superimposed on the surface of the image forming body so as to form a color image; and the color image is transferred onto a transfer sheet so as to record the color image.
A large number of methods and apparatus have been proposed to provide color images by means of electrophotography. For example, the following method has been disclosed in Japanese Patent Publication Open to Public Inspection No. 100770/1986: operations of latent image formation and development; are conducted on the surface of a photoreceptor drum in accordance with the number of color separation of a document image; the formed image is transferred onto the surface of a transfer drum each time the latent image is developed so that a multicolor image is formed on the transfer drum; and then the image is transferred onto a transfer sheet to obtain a color copy. Dimensions of the aforementioned apparatus are increased and further the structure becomes complicated because it is necessary to provide to the apparatus not only a photoreceptor but also a transfer drum, the circumferential surface of which is sufficiently wide to accept an image thereon.
According to Japanese Patent Publication Open to Public Inspection No. 149972/1986, the following method has been disclosed: operations of latent image formation and development are conducted on the surface of a photoreceptor drum in accordance with the number of color separation of a document image; the image is transferred onto a transfer sheet each time it is developed so as to provide a multicolor image. In this method, multicolor images can not be superimposed with high accuracy. Therefore, it is impossible to provide images of high quality.
Also, the following method has been disclosed: operations of latent image formation and development by color toner are repeatedly conducted on a photoreceptor drum so as to superimpose color toner images on a photoreceptor; and then the superimposed image is transferred onto a transfer sheet to provide a color image. The essential process of this multicolor image forming method has been disclosed by the inventors in Japanese Patent Publication Open to Public Inspection Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 15847/1985.
A method in which color toner images are superimposed on a photoreceptor is advantageous in that color images of high quality can be provided, however, it is necessary to rotate the photoreceptor drum a plurality of times in order to obtain one color image. Therefore, the copy cycle is extended.
Therefore, the efficiency of one copy cycle is improved in the following manner: the length of the photoreceptor drum circumference is determined so that the width of an image can be accommodated in the circumference in order to minimize the number of revolutions of the photoreceptor drum when toner images are superimposed; and further a transfer operation to transfer the color toner image onto a transfer sheet and a cleaning operation to clean the surface of the photoreceptor after transfer are conducted in the same rotational cycle of the photoreceptor drum.
However, in the case where the photoreceptor drum is cleaned in the same rotational cycle as that of a transfer operation, image quality is deteriorated because the transfer operation is affected when the rotational speed of the photoreceptor temporarily fluctuates being influenced by the contact pressure of the cleaning member.
The present invention has solved the aforementioned problems. It is a primary object of the present invention to provide a color image forming apparatus in which image quality is not deteriorated even when a cleaning operation is conducted on the photoreceptor drum so that color images of high quality can be always obtained effectively.